KIf4 (GKLF) and Klf5 (IKLF) are members of the Kr[unreadable]ppel-like factor (KLF) gene family and are expressed in the epithelia of the gastrointestinal tract as well as several other organs. Members of the KLF family have important roles in development, cell proliferation, and differentiation. In vitro, KIf4 has been implicated in growth arrest and down-regulation of cell proliferation, while KIf5 has been proposed to positively regulate cell proliferation. KIf4 is expressed exclusively in the differentiated compartment of the intestine while KIf5 expression is confined to the proliferative compartment. Mice homozygous for a null mutation in KIf4 die within 15 hours after birth and show selective perturbation of epithelial cell differentiation in the epidermis and colon. Since KIf4-/- mice die universally on postnatal day 1, nothing is known about the impact of the loss of KIf4 in the colon later in development. The function of KIf5 in the gastrointestinal tract has not yet been investigated. We will elucidate the roles of KIf4 and KIf5 in gastrointestinal proliferation and differentiation in vivo by testing the following hypotheses: (1) Mice lacking KIf4 in the gastrointestinal epithelium will have severe perturbations in epithelial proliferation and/or differentiation in adulthood; (2) Mice with overexpression of KIf5 in the gastrointestinal epithelium will have abnormal epithelial proliferation and/or differentiation; and (3) KIf4 and KIf5 regulate proliferation and/or differentiation in the gastrointestinal tract by modulating the expression of specific factors in the epithelium. The following specific aims will be pursued: (1) To analyze the function of KIf4 in gastrointestinal proliferation and differentiation in adult mice by morphological analyses of intestine-specific knockouts; (2) To analyze the function of KIf5 in gastrointestinal proliferation and differentiation in vivo using the bigenic GAL4/UAS system to overexpress KIf5 in the murine gastrointestinal epithelium; and (3) To identify other factors involved in gastrointestinal proliferation and differentiation in vivo using microarray analysis of KIf4 null mice, intestine-specific KIf4 knockouts, and KIf5 transgenic mice.